There is at least one group of Christian sects that does not believe that Paul's writings are the divinely inspired Word of God: the various "New Church" or "Swedenborgian" denominations that accept the theology and Bible interpretations of Emanuel Swedenborg (1688–1772).
Swedenborg listed the books that he considers to be the Word of God in three places: Arcana Coelestia ("Secrets of Heaven") #10325, The New Jerusalem and its Heavenly Doctrine #266, and The White Horse #16. These lists do not include the Acts and the Epistles in the New Testament. For the full list, and Swedenborg's criteria for inclusion in his canon of Scripture, see this question and its accepted answer: What writings are held as "biblical canon" by Swedenborgians?
In a letter to Dr. Gabriel Beyer dated April 15, 1766, Swedenborg explained why he did not quote from Paul's letters in his first and largest published theological work, Arcana Coelestia, providing at the same time an explanation of why he did not consider them to be part of the Word of God:
In respect to the writings of the apostles and Paul, I have not quoted them in Arcana Coelestia, because they are doctrinal writings, and consequently are not written in the style of the Word, like those of the prophets, of David, of the Evangelists, and the Book of Revelation. The style of the Word consists altogether of correspondences, wherefore it is effective of immediate communication with heaven; but in doctrinal writings there is a different style, which has indeed communication with heaven, but mediately. They were written thus by the apostles, that the new Christian Church might be commenced through them; wherefore matters of doctrine could not be written in the style of the Word, but they had to be expressed in such a manner, as to be understood more clearly and intimately. The writings of the apostles are, nevertheless, good books of the church, insisting upon the doctrine of charity and its faith as strongly as the Lord Himself has done in the Gospels and the Book of Revelation; as may be seen and found evident by every one who in reading them directs his attention to these points. ("Third Letter of Emanuel Swedenborg to Dr. Beyer," reproduced in English translation from the original Swedish in Documents Concerning Swedenborg, R.L. Tafel, Ed. London: Swedenborg Society, 1877, Volume II, Part I page 240.)
"Correspondences" refers to Swedenborg's doctrine of symbolic, metaphorical, and spiritual interpretation of the Scriptures. His view was that the books that are part of the Word of God have an inner meaning that speaks of the Lord (Jesus Christ), his kingdom, and the church (considered broadly as all people of faith who live a life of love toward the neighbor), which is the Lord's kingdom on earth.
As stated in his letter to Beyer quoted above, Swedenborg did not see the Acts and the Epistles, including the letters of Paul, as having that deeper spiritual and divine meaning. He saw them, rather, as historical and doctrinal writings intended to inform and instruct the nascent Christian Church. He therefore saw them as good books for the Christian Church, but not as the Word of God.
I am not aware of any other Christian sects that do not consider Paul's writings to be the divinely inspired Word of God. But see the related question (still unanswered as of now): Do any non-Swedenborgian Christian denominations have a smaller biblical canon than Protestantism?